Field of the Invention
The invention relates to a wireless receiver that receives a radio wave from, for example, a wireless microphone and demodulates an audio signal, and in particular relates to a wireless receiver capable of preventing fluctuation of a reception reaching distance (which determines an area where a combination system of a wireless microphone and a receiver can be used) due to temperature dependence of the receiver.
Description of the Related Art
For example, in a wireless receiver that receives a modulated RF signal (hereinafter referred to as an RF signal) transmitted from a wireless microphone and demodulates an audio signal, audibly unpleasant noise is generated when transmission of the RF signal (in particular Frequency-Modulated RF signal, hereinafter referred to as FM-RF signal) from the wireless microphone is stopped, or in a case where a reception state of the RF signal is deteriorated.
In order to remove such noise, this kind of wireless receiver includes a muting circuit (also referred to as a squelch circuit).
As one of the muting circuits, an RF muting circuit has been proposed that opens and closes a signal line of a demodulated signal according to a level of the received RF signal. The RF muting circuit operates to close the signal line of the demodulated audio signal and to reduce output of the audio signal when the level of the received RF signal is below a predetermined threshold value.
That is, to operate the RF muting circuit, a unit that obtains information (received signal strength indication: RSSI) according to the level of the received RF signal, and a comparison unit that compares a value of the RSSI with a predetermined threshold value are included.
However, the value of the RSSI largely depends on temperature of a front-end including an RF amplification circuit and a local oscillation circuit, and the like. Consequently, with the above-described muting circuit using the RSSI having the temperature dependence, an operating point of the muting circuit naturally fluctuates due to the temperature. For this reason, a reception reaching distance (which determines an area where the system can be used) of the combination system of the wireless microphone and the wireless receiver fluctuates (varies) due to operating temperature of the receiver.
In order to reduce such temperature dependence, a wireless receiver has been proposed in which a countermeasure is applied such as correction of the value of the RSSI corresponding to the operating temperature, and the receiver has been disclosed in, for example, JP 2006-270902 A (Patent Document 1) and JP 2014-017782 A (Patent Document 2).
The receiver disclosed in JP 2006-270902 A uses a constant current circuit in which an output current value is changed according to the temperature and corrects the temperature dependence of the RSSI, thereby suppressing variation of the operating point of the muting circuit according to the temperature.
The receiver disclosed in JP 2014-017782 A arranges, in the front-end, a correction circuit including a thermistor which changes its gain depending on the temperature, thereby correcting the temperature dependence of the RSSI similarly, and suppressing variation of the operating point of the muting circuit.
That is, both receivers correct the output of the RSSI having the temperature dependence with the constant current circuit or the circuit including the thermistor both having the temperature dependence similarly, and apply the value (voltage value) of the corrected RSSI to the comparison circuit, and compare the value with a fixed mute reference voltage (threshold voltage).
FIG. 8 is a graph describing mute operation in the conventional receiver, and the left vertical axis indicates the mute reference voltage, the right vertical axis indicates the RSSI voltage, and the horizontal axis indicates the operating temperature. A characteristic denoted by a indicates an output characteristic of the RSSI before correction of the temperature dependence, and a characteristic denoted by b indicates an example of an output characteristic of the RSSI after correction of the temperature dependence. A dashed line denoted by c indicates the fixed mute reference voltage (threshold voltage).
By the way, it is very difficult to perform correction so that the temperature characteristic is completely eliminated for the characteristic a of the RSSI before the correction, by using the constant current circuit or the like as disclosed in JP 2006-270902 A and JP 2014-017782 A, within a practical temperature range in which this kind of wireless receiver is used. That is, there are many examples in which the value of the RSSI becomes a gradient curve as illustrated by the corrected output characteristic b.
In this example, compared to an execution temperature t1 of the mute operation of the output a before the correction, an execution temperature of the mute operation of the output b after the correction increases to t2, and the reception reaching distance extends substantially. However, in the output b after the correction, the mute operation is executed at the temperature t2, but the mute operation is not executed at the temperature t1, and the reception reaching distance (which determines an area where the system can be used) fluctuates due to the temperature.